Determining properties of magnetic elements

ABSTRACT

A method of determining differing characteristics of magnetic dipole elements such as orientation, coercivity, bias and response amplitude and a tag reader for reading magnetic tags containing such elements. The elements are scanned by a rotating magnetic field and two sets of transition data are determined. The transition data sets are associated with respective elements and analyzed to determine mean field values resolved along the element vectors. These field values are used to determine properties of the elements, such as coercivity.

This application is the U.S. national phase of international applicationPCT/GB00/03092 filed 11 Aug. 2000, which designated the U.S.

FIELD OF THE INVENTION

This invention relates to magnetic elements, particularly but notexclusively to methods of distinguishing between magnetic elements andmethods and apparats for reading magnetic data tags which include one ormore magnetic elements, each of which can differ in coercivity,saturated dipole moment (i.e. response amplitude), orientation and biasfield.

BACKGROUND

Co-pending PCT publication number WO99/35610 describes tags and readersystems primarily intended for tags fabricated from magnetic material oflow coercivity, with elements at different orientations, in which datais recorded primarily by means of the orientation of the elements withrespect to each other. The described system assumes that thecoercivities of the tag elements are all the same, and arc very smallcompared to the interrogation field.

SUMMARY OF THE INVENTION

According to the invention, there is provided a method of reading amagnetic tag having at least one magnetic element, comprisinginterrogating the tag with a scanning magnetic field, determiningtransition data associated with changes in the magnetisation state ofthe at least one magnetic element, associating the transition data withone or more respective elements; and for each element, determining theelement direction which corresponds to the transition data for thatelement.

Preferably, the element direction is determined by selecting thedirection that minimises the scatter of the transition field vectorsresolved along the direction of the element.

The transition data for each element can be grouped into two sets, whichcan be referred to as forward and reverse transitions. All those in theforward transition group have a positive component of the field vectordH/dt along the element vector, and all those in the reverse group havea negative component of dH/dt along the element vector. Mean fieldvalues, resolved along the element vector, can be calculated. Thecoercivity of the element is then calculated as half the differencebetween the forward and reverse mean values, and the bias field alongthe element is calculated as the sum of the forward and reverse meanvalue

According to the invention, there is further provided a method ofdistinguishing between a plurality of magnetic elements, comprising thesteps of applying a scanning magnetic field to the elements, determiningthe direction of each of the elements, for each of the elements,determining the components of the field in the direction of the elementat which the element switches magnetisation states; and from saidcomponents, determining, for each of the elements, respectivecharacteristics of the element.

The invention further provides a method of determining, for a magneticelement, any one or more of a plurality of characteristics comprisingthe coercivity of the element, the local magnetic field bias resolved inthe direction of the element and the orientation of the element,comprising the steps of applying a varying magnetic field to theelement, determining the direction of the element, determining tiecomponents of the field in the direction of the element at which theelement switches magnetisation states; and from said components,determining the one or more characteristics of the element.

According to the invention, there is also provided a magnetic tag readerfor reading a magnetic tag having at least one magnetic element,comprising means for interrogating the tag with a scanning magneticfield, means for determining transition data associated with changes inthe magnetisation state of the at least one magnetic element, means forassociating the transition data with one or more respective elements;and means for determining, for each element, the element direction whichcorresponds to the transition data for that element

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a magnetic data tag reading system;

FIG. 2 is a schematic diagram showing the components of the magneticdata tag reading system of FIG. 1 in more detail;

FIG. 3 is a schematic diagram showing details of the signalprocessor/controller illustrated in FIGS. 1 and 2;

FIGS. 4 and 5 illustrate the receive coil set;

FIGS. 6 and 7 illustrate the transmit coil set;

FIG. 8 illustrates the antenna comprising transmit and receive coilsets;

FIG. 9 is a flow chart illustrating the overall processing algorithm;

FIG. 10 is a schematic diagram of data acquisition circuitry,

FIG. 11 illustrates the transmit current waveforms;

FIG. 12 illustrates a flow chart for the signal processing and filteringalgorithm;

FIG. 13 illustrates signals at the inputs to the ADC from the x, y and zreceiver coil preamplifiers, for a single element transition;

FIG. 14 illustrates the composite filter output of the signal in FIG.13;

FIG. 15 illustrates a 3D scatter plot of the filtered receiver vectors;

FIG. 16 illustrates a flowchart for the clustering algorithm used forplanar tags;

FIG. 17 illustrates the composite filter output for three parallelelements;

FIG. 18 illustrates a flowchart for a parallel element clusteringalgorithm,

FIG. 19 illustrates a 3D scatter plot of the transition field vectorsfor a single element;

FIG. 20 illustrates the same 3D scatter plot as FIG. 19, tilted suchthat the transition planes are edge-on;

FIG. 21 illustrates distribution of field vectors that occur along amisaligned element direction vector; and

FIG. 22 illustrates a flowchart for the calculation of the meanswitching field, switching field variance, coercivity and DC bias field.

DETAILED DESCRIPTION

Referring to FIG. 1, a magnetic tag reading system comprises a magneticdata tag 1, an interrogation unit 2 and a signal processor/controller 3.Magnetic tags 1 to be used with a magnetic tag reader according to theinvention can record information by means of elements of differingcoercivities, local bias fields and response amplitudes, as well asorientation This includes tags described in PCT publication numberWO99/35610, as well as tags described in, for example, U.S. 5,204,526,U.S. 5,729,201 and WO98/26312. In general terms, magnetic tags 1comprise magnetic elements which typically switch magnetisation state,for example magnetisation direction, at given values of applied fielddepending on element properties, for example coercivity. These elementsinclude, for example, thin film elements, bistable elements, Barkhausenwire elements and high-permeability elements. The applied field whichcauses switching depends on the magnitude of the component of theinterrogation field vector in the direction of the element

Referring to FIG. 2, the tag 1 is attached to an item being labelled ortagged 4, and is placed within an interrogation volume 5 within theinterrogation unit 2. The interrogation unit 2 includes an antenna 6,which comprises transmit and receive coil sets 7, 8. The tag 1 isinterrogated by a scanning magnetic field 9 generated by the transmitcoil set 7 under the control 10 of the processor/controller 3. Inresponse to the interrogating magnetic field 9, the tag 1 generates adetectable magnetic field response 11, which is detected by the receivecoil set 8. The processor/controller 3 receives input signals 12, 13from the transmit and receive coil sets 7, 8 respectively and processesthe signals to decode data stored on the tag, which is made available atan output 14.

Referring to FIG. 3, the processor/controller 3 comprises a waveformgenerator for driving the transmit coil set 7, data acquisitioncircuitry 16 for receiving respective input signals 12, 13 from thetransmit and receive coil sets 7, 8 and a digital signal processor 17for processing the resulting output signals 18 from the data acquisitioncircuitry 16 to provide the decoded tag data 14.

The transmit and receive coil set arrangement 7, 8 is described indetail by reference to FIGS. 4 to 8.

FIGS. 4 and 5 illustrate the receive coil set 8. The receiver coils areconstructed on a cylindrical former 20 of diameter 200 mm and length400mm. FIG. 4 illustrates the three sets of orthogonal coils used tocouple with the tag magnetic elements within the interrogation zone. Forthe y-direction, the receiver coil set comprises 4 coils 21, 22,23, 24.Inner coils 22, 23 lie on the former 20 and extend 120mm along thex-direction 25. Both inner coils 22, 23 comprise 100 turns 0.4 mm ecw.The outer coils 21,24 comprise 58 turns of 0.4 mm ecw and are wound on asecond co-axial former (not shown) 260 mm in diameter. The coils extend156 mm along the x-direction. The four coils 21, 22, 23, 24 areconnected in series in the electrical sense illustrated and ‘balanced’by small mechanical re-alignments to achieve zero sensitivity to auniform magnetic field. A second receiver coil set as illustrated issensitive to tag generated field in the z-direction. This coil set isidentical to the coils 21, 22, 23, 24 but rotated through 90° as shown.The third coil set sensitive to tag generated field in the x-directioncomprises two solenoid coils 26, 27. The inner coil 26 comprises 100turns 0.4 mm ecw wound on the former 20, and is 120 mm long. The outercoil 27 comprises 58 turns of 0.4 mm ecw wound on the second 260 mmdiameter co-axial former and is 156 mm long. FIG. 5 illustrates all thecoils wound on the inner former 20, and the outer former 28.

FIG. 6 illustrates the three orthogonal transmit coils configuration 7.The coils are wound on a cylindrical former 30, 370 mm long and 300 mmdiameter. A uniform magnetic field in the y-direction is produced byfour coils 31, 32, 33, 34. First and third coils 31, 33 comprise a‘modified Helmholz’ arrangement similar to coils 15 and 16. Second andfourth coils 32, 34 comprise a second modified ‘Helmholz’ arrangement,with a magnetic axis 25° offset from the first and third coils 31, 33.The two ‘modified Helmholz’ coil sets have magnetic axes 12.5° eitherside of the y-direction. The first coil 31 comprises 50 turns 1.4 mm ecwand extends 370 mm in length along the former. Where this coil 31connects across the open end of the former 30, the coil is a flattenedhalf circle with the total coil aperture width of 570 mm. The two edgesof the coil 31 that lie along the solenoid (x-direction) subtend 120° atthe axial centre of the former. Second to fourth coils 32, 33, 34 areidentical in size and form. Their orientation around the former 30 isdescribed above. The four coils are connected in series in the senseillustrated. A second transmit coil set generates uniform field in thez-direction. This set comprises four identical coils orientated in anorthogonal direction as illustrated. The final transmitter coil consistsof a long solenoid coil 35 comprising 260 turns of 1.4 mm ecw on thecoil former. This generates uniform field in the x-direction.

FIG. 7 shows the overall transmit coil arrangement for generatinguniform field in three orthogonal directions. FIG. 8 illustrates theantenna 6. The transmit coils on the former 30 are located co-axiallywith the receiver coil tube 20. The interrogation volume 5 is defined bya further 190 mm ID co-axial tube (not shown) that is used to define amechanical constraint on possible tag positioning in the antenna 6. Thelongitudinal region of highest sensitivity is less than 10 cm long andtags can be accurately read when separated by 10 cm or more along theaxis of the reader tube.

FIG. 9 illustrates the overall sequence of steps required to decode datastored on a magnetic tag. The first stage is data acquisition (step s1).Data is acquired by detecting the field 11 resulting from theapplication of a scanning interrogation field 9 to the tag 1, digitisingthe resulting signals and storing them for subsequent processing. Thisresults in 3 channels of input data, one for each of the x, y and zdirections. Digital signal processing is carried out to identifyindividual switching points, also referred to herein as transitions(step s2). This results in an array of transition information. Eachtransition is associated with an element (step s3) to provide an arrayof elements. The elements are then individually decoded (step s4).Finally, the tag is decoded to provide tag value data (step s5).

Data Acquisition

Referring to FIG. 10, an example of the signal processor/controller 3according to the invention comprises a National Instruments PCI67114-channel DAC card 39 for waveform generation and a National InstrumentsPCI6110E 4-channel ADC card 40 for data acquisition. The cards aremounted into an industry standard IBM compatible PC 41 running Windows95™. The waveform generation card 39, under software control, generatesthree transmit excitation voltages 42, 43, 44 which are passed throughrespective low-pass filters 45, 46, 47 and amplified by respective poweramplifiers 48, 49, 50 to drive respective orthogonal transmit coils,which are arranged in a series resonant configuration with respectivecapacitors 51, 52, 53 and resistors 54, 55, 56. The drive current is forexample 3A rms to generate a 2.5 kA/m interrogation field Thetransmitter currents are monitored by respective current sense resistors57, 58, 59 which are fed through respective amplifiers 60, 61, 62 asinputs 63, 64, 65 to the data acquisition card 40, where they aredigitised at a sample rate of, for example, 160 kH The instantaneoustransmit field vector can be determined from these three signals withknowledge of the relationship between the transmit coil field and thecurrent response. For example, pre-calibration of the system is carriedout by measuring the transmit field for different values of drivingcurrent.

Signals induced in the orthogonal receive coils are amplified byrespective amplifiers 66, 67, 68, filtered by respective 130 Hz notchfilters 69, 70, 71 to remove any transmit field component, and fed asinputs 72, 73, 74 to the data acquisition card, where they are digitisedat a sample rate of, for example, 160 kHz. The data acquisition isbuffered in such a fashion that data is clocked into a buffer, and readfrom the buffer asynchronously at some later point. The buffer depth issufficient to accommodate the worse-case latency in the subsequentprocessing step.

A continuous scan is used in this example to interrogate the tag, basedon a nominal 130 Hz rotating magnetic field, whose normal vector isarranged to trace out a spiral scan over the surface of a completesphere, tracing a path from one pole of the sphere to the other andback. The equations for the components of the ‘transmitted’ Binterrogation field are given by:

B _(x)=(cos²(φ)*cos(θ)+sin²(φ))*cos(ωt)+(sin(φ)*cos(φ)*cos(θ)−cos(φ)*sin(φ))*sin(ωt)

B _(y)=(cos(φ)*sin(φ)*cos(θ)−sin(φ)*cos(φ))*cos(ωt)+(sin²(φ)*cos(θ)+cos² (φ))*sin(ωt)

B _(z)=(−cos(φ))*sin(θ)*cos((ωt)−sin(θ)*sin(φ)*sin(ωt)

where t is the time, ω is the angular frequency of the 130 Hz scan,φ=(constant)* θ, and θ=cost⁻¹(1−t/T). T is the total time for onecomplete interrogation.

FIG. 11 shows the three transmit current waveforms 80, 81, 82 receivedat the inputs 63, 64, 65 of the data acquisition card 40.

Digital Signal Processing

The digital signal processing stage (step s2) performed on the datainput to the data acquisition card 40 is now described with reference tothe flow chart description of the processing algorithms in FIG. 12.

The purpose of the DSP algorithm is to identify individual transitions,and to record all the relevant parameters on each transition forsubsequent processing algorithms. This leads to a large reduction in thevolume of data passed on to the subsequent processing stages.

The DSP algorithm operates on the three channels of sample data producedby the, data acquisition process. FIG. 13 shows the raw impulseresponses in the x, y and z channels for a single transition.

Referring to FIG. 12, in a first step s10, an FIR filter is applied toall three channels to produce three sets of filtered data, which form areceiver vector. The simplest filter consists of three rectangularsections, and provides a method of measuring the height of the peaks inthe raw data. If the central section has width w and height+1, then theouter two sections have width w/2 and height−1. The width, w, istypically the same value as the response time of the magnetic element,for example, 20-30μs.

The transmit field vector, H, is used to determine the correct polarityof the element transition in each of the x, y and z receiver coils. Thetransition polarity in any receiver coil is directly related to thepolarity of the rate of change of the field vector dH/dt in thedirection of the receiver coil. dH/dt values are used to produce a“polarity vector”, where each component can take the value±1. The scalar(dot) product of the polarity vector with the filtered receiver vectoris calculated (step s11) and the receiver vector magnitude, a positivenumber, is multiplied by the sign (±1) of the result (step s12). Thisresults in the composite signal shown in FIG. 14, in which the polarityof transitions for every element is always the same, allowing the use ofa simple peak detector to determine peak values.

Peak detection techniques are well known in the art. In this case, asimple threshold is used to gate the peak detector input data, to avoidnoise appearing as spurious peaks. A peak is identified when three ormore values exceed the threshold (step s13), and where the current valueis greater than both the previous and next value (step s14) The time ofthe peak is interpolated to a greater resolution than the samplefrequency by a simple quadratic fit to these three points.

The data for each transition is stored in an array (step s15). The dataincludes:

Time

Field vector (H)

Rate of change of field vector (dH/dt)

Receiver vectors (both raw and FIR filtered)

Element Association

The function of the element association algorithm is to associatetransition data points with particular magnetic elements in the tag.Subsequent processing steps can then analyse the data for each magneticelement in isolation, thereby reducing an apparently complex problemwith multiple elements into a series of relatively simple numericalsolves.

There are two primary mechanisms that are used to associate transitionswith elements, depending on whether the elements are generally parallelor not. These are described below. In the general case, the first stepis to separate into groups using a non-parallel algorithm, and then, ifrequired, to analyse each separate group to see if it contains more thanone parallel or near-parallel elements.

For non-parallel elements, the filtered receiver vectors are used toseparate out the transitions between elements. This can be clearly seenfrom FIG. 15, which shows the filtered receiver vectors in a 3D scatterplot for an example tag having seven non-parallel elements. Inspectionof the plot shows that the majority of the transition points Hie alongone of 7 different lines through the origin, which indicates that thereare 7 discernible directions of elements in the example tag. Eachdirection can be described by two parameters, and therefore thetransitions can be clustered together into groups in 2D. There are anumber of different appropriate techniques than can be used to achievethis multi-dimensional clustering (e.g. S. Makeig, S. Enghoff, T -P.Jung, M. Westerfield, J. Townsend, E. Courchesne and T. J. Sejnowski,“Moving-Window. Independent Component Analysis of Event-Related EEGData: Component Stability, Journal of Neurophysiology”). Additionalknowledge about the particular tag construction can be useful tosimplify the problem. For example, if all the elements are in the sameplane, then the problem can be reduced to a one-dimensional problemKnowledge of the number of elements expected can assist in theclustering process.

In the particular case of a planar tag, with a known number of elements,the algorithm outlined in FIG. 16 is used. The normal to the plane ofthe transitions is determined by, for example, a numerical process (steps20). For example, the dot product of every receiver vector with anestimated direction vector is calculated, and this process is iterateduntil the sum of the magnitude of the dot products is minimised. Thisreduces the problem to a 1-D problem i.e. the angle in the plane. Anin-plane set of vectors can be calculated from the original set ofvectors simply by subtracting from each vector in turn the dot productof itself with the normal to the plane. In-plane angles between any twoin-plane vectors can them simply be calculated in the usual way usingdot products. All the in-plane angles are wrapped into the range 0-180°by adding or subtracting multiples of 180° as required. The algorithmcalculates a histogram of in-plane angles relative to some arbitrarydatum, such as the first point. For example, if the histogram bins are1° wide, then the nth bin will contain a count of the number of anglesthat fall in the range n° to (n+1)°. This will typically give a seriesof peaks, one for each element. For example, the algorithm obtains thesecond point from the transition array (step s21), measures the in-planeangle relative to the first point (step s22) and increments theappropriate bin of the histogram (step s23). This process is repeateduntil all the data has been processed (step s24). After applyingGaussian smoothing to the histogram data (step s25), the direction of anelement in the tag can be found by determining the highest peak in thishistogram (step s26). To determine the transitions that belong to theelement in a given direction, the algorithm finds all the transitionsthat are within, say, 2° (in plane) of this direction (step s27). Theprocessed peak is then eliminated from the calculation (step s28) andthis processing sequence is repeated (steps s26 to s28) until all thedata has been processed (step s29).

To separate parallel elements, the algorithm makes use of two propertiesof a continuous scan of the field vector, H, around the elements, firstthat the elements transition in order from the lowest to the highestcoercivity and second, that the field vector, H, rotates by at least 90°between the last transition of the highest coercivity element in onedirection and the first transition of the lowest coercivity element inthe reverse direction.

If some of the elements do not change state, because the transmit fielddoes not reach a high enough value, then there will be fewertransitions, in a 180° scan, than there are elements. In this case,transitions are “lost”, starting with the highest-coercivity element.FIG. 17 shows the filtered composite waveform over a few rotations oftransmit field, for three parallel elements with different coercivities.Each element is associated with a respective peak 90, 91, 92 and it isrelatively straightforward to separate out the transitions belonging todifferent elements. An outline algorithm to achieve this is shown inFIG. 18.

This works by maintaining an element counter that is incremented eachtime a new transition is identified, and set to zero each time the fieldrotates by more than 90° between transitions. Data is extracted from thetransition array (step s30) and the algorithm determines whether thetransmit field has rotated by more than 90° since the last transitionpoint (step s31). If it has, the element counter is reset to zero (steps32). Following this, the element number corresponding to the transitionis set according to the current value of the element counter (step s33).The element counter is then incremented (step s34) and the processrepeated for the next transition point (step s30). For example, thefirst transition following the zeroing of the element counter isassociated with element 0, the next with element 1 and so on, until thefield has rotated by more than 90 degrees.

Element Decode

The purpose of the element decoding algorithm is to take transition databelonging to one element, and to determine the best-fit direction vectorfor this element. Once the direction is known, the coercivity of theelement, and any net DC field or “bias” along the element vector can becalculated.

FIG. 19 is a 3D scatter plot of transition point field vectors, for asingle bistable magnetic element with a finite coercivity. In thisexample, the field has been scanned approximately over the surface of asphere, so the transition points lie roughly on two circles 93, 94. Moregenerally, the transition points would be expected to lie on one of twoplanes. By tilting the view of the transition data in the scatter plot,it is possible to show the two planes edge-on, as illustrated in FIG.20. The bold vertical arrow 95 shows the element vector.

The element decoding algorithm attempts to determine the best vectordirection for the element, by minimising the scatter of field vectorsresolved in this direction. FIG. 21 illustrates the situation where aguess 96 has been taken for the element vector that is not in thecorrect direction. Taking the upper set of transitions 93, it is clearthat when these are projected onto the element vector 96, they form anextended distribution 97 (shown by a darkened section) along the vector96. As the vector is rotated around, the extent of this distributionwill be smallest when the vector is closest to the actual direction ofthe tag element.

FIG. 22 shows a flowchart for the algorithm used to calculate the errorused in the iterative solving process. The algorithm uses the currentguess for the element vector direction, V. Initially, this is the vectordirection from the element association algorithm.

The first data point is retrieved (step s40) and the dot product of thedirection estimate V with the polarity vector for dH/dt is calculated,as described in relation to FIG. 12 above (step s41). If this ispositive (step s42) i.e. for the upper set of transitions, then the dotproduct of the field vector and the element vector is calculated (steps43) and added to the upper set of statistics (step s44). The dotproduct resolves the component of the field vector along the elementvector. The same calculation is carried out for the lower set oftransitions (steps s45, s46), indicated by the negative dot product atstep s41. The upper and lower (forward and reverse) sets of transitionsare distinguished by the sign of dH/dt along the direction of theelement, or alternatively by the sign of the filtered receiver vectoralong the direction of the element. This procedure is repeated for allthe data points (step s47). An average value of variance is calculatedfrom the variances for each of the upper and lower sets of transitionsseparately, weighted by the number of transitions in the upper and lowersets of transitions (step s48). A standard formula is used to calculatethe variance for each set of data. For a set of measured data points, x,the variance, var(x) is the mean of the squares of x minus the square ofthe mean of x, or mathematically var(x) = ⟨x²⟩ − ⟨x⟩²

The weighted variance of N_(w) upper transition points, u, with variancevar(u) and N_(t) lower transition points, l, with variance var(l) isthen given by:${var} = \frac{{N_{u}{{var}(u)}} + {N_{l}{{var}(l)}}}{N_{u} + N_{l}}$

The weighted variance is used as a measure of the error in the guessedvector direction. When the guessed direction is equal to the actualelement direction, the weighted variance will generally have its minimumvalue. The value will never fall to zero, because there is always acertain amount of noise in the determination of the transition field,arising from sources such as electronic noise and randomness in thematerial behaviour. In the simplest case, the variance is a function(numerically evaluated, rather than an analytic function) of twodirection variables, such as θ and φ from the spherical polarco-ordinates (r, θ, φ). The value of this function can be minimisedusing a standard numerical minimisation algorithm. The variance variesapproximately quadratically with the deviation from the ideal direction,and this means that the minimisation algorithm can be extremelyefficient (the “quadratic” case is generally considered to be theeasiest). Multi-variate numerical minimisation algorithms are well knownin the art—for example, Powell's method

When the weighted variance is not minimum (step s49), the directionestimate V is adjusted in accordance with the appropriate minimisationalgorithm (step s50) and the algorithm re-run with the new value of V.When the weighted variance is minimised, the mean values of the fieldfor the upper and lower sets of transitions are calculated (steps s51,s52). The coercivity of the element is calculated as half the differencebetween the two switching fields (step s53), while the DC field alongthe element is calculated as the sum of the two switching fields (steps54).

Additional parameters as well as direction can usefully be added to thenumerical minimisation. The most important term to add is the vectorvelocity, which allows the algorithm to deal with the movement of thetag elements during the decoding process. The element direction in stepss41, s43, s45 is then a function of the time at which the transitionoccurs, and the function for the variance then depends on fourparameters (for example θ, φ, dθ/dt and dφ/dt). Once again, thisfunction can simply be minimised using a standard multi-variateminimisation algorithm

Many magnetic elements do not behave ideally, and show significantchanges in their switching field (or coercivity) depending on the valueof dH/dt. A general form for the switching field, H_(switch), resolvedalong the element is:$H_{switch} = {H_{0} + {k_{a}\left( \frac{H}{t} \right)}^{a} + {k_{b}\left( \frac{H}{t} \right)}^{b} + \ldots}$

where a,b etc are arbitrary powers. If the coefficients k are known,then the value of H₀ can be calculated from the measured switchingfield, and the variance of this value can be minimised as before. If thecoefficients k are not known, but the values a, b are known, then thefunction for the numerical minimisation can also include thecoefficients, k, as function arguments, as well as the direction andvelocity terms. In this case, the values of k can be used to distinguishbetween different types of materials and thereby store more data.

Anisotropic thin-film magnetic materials can exhibit a further form ofnon-ideal behaviour. For materials with an easy-axis of magnetisation,the in-plane field perpendicular to the easy axis can influence thefield at which the material switches. A similar approach to the onedescribed above can be used to calculate a nominally constant value, H₀,from the raw switching points.

Tag Decode

The primary output data for each magnetic element from a readeraccording to the invention is as follows:

Orientation in the reader (vector)

Coercivity of the element (scalar)

Bias field along each element (scalar)

Amplitude response (scalar)

Secondary data for each element includes

dH/dt coefficients

Perpendicular field coefficients

Response time

Characteristic response “shape” or spectrum

Statistical distribution of primary parameters

This data assumes little about the construction of the tag. Thestructure of the tag (e.g. which elements share bias magnet elements)may be used to provide more detail—for example, the magnitude anddirection of an overall bias field. The details of the chosen codingscheme are used to translate all these raw parameters into useful datastored on the tag.

The above examples of the invention are intended to be illustrative,rather than restrictive. A person skilled in the art would understandthat various modifications and variations in the detailed implementationare possible, and are considered to be within the scope and spirit ofthe invention as defined in the appended claims.

What is claimed is:
 1. A method of reading a magnetic tag having atleast one magnetic element, comprising; interrogating the tag with ascanning magnetic field; determining transition data associated withchanges in the magnetisation state of the at least one magnetic element;associating the transition data with one or more respective magneticelements; and for each magnetic element, determining the elementdirection which corresponds to the transition data for that magneticelement.
 2. A method according to claim 1, wherein the step ofdetermining the magnetic element direction comprises selecting thedirection which minimises the scatter of transition field vectorsresolved along the direction of the magnetic element.
 3. A methodaccording to claim 1, including grouping the transition data by the typeof magnetic element transition.
 4. A method according to claim 3,comprising grouping first and second types of magnetic elementtransition.
 5. A method according to claim 4, wherein the first type ofmagnetic element transition comprises a forward transition and thesecond type of magnetic element transition comprises a reversetransition.
 6. A method according to claim 4, wherein a signal defininga transition is received by one or more receiver coils, includingdetermining the type of transition in accordance with the polarity ofthe rate of change of the field vector in the direction of the magneticelement.
 7. A method according to claim 4, including determininginformation relating to the switching fields for each of the first andsecond types of transition.
 8. A method according to claim 7, comprisingdetermining magnetic element characteristics from said switching fieldinformation relating to transition data associated with a magneticelement.
 9. A method according to claim 8, further comprisingcalculating the coercivity of the magnetic element as substantially halfthe difference between first and second switching fields.
 10. A methodaccording to claim 8, further comprising calculating the bias field onthe magnetic element as substantially the sum of first and secondswitching fields.
 11. A method according to claim 9, wherein the firstswitching field comprises the mean value of the switching fields for thefirst type of transition and the second switching field comprises themean value of the switching fields for the second type of transition.12. A method according to claim 1, including associating the transitiondata with one or more respective magnetic elements using a receivervector whose components represent the amplitudes of the signals in oneor more receive coils.
 13. A method according to claim 1, comprisingscanning the tag using a rotating magnetic field.
 14. A method accordingto claim 13, in which the tag comprises a plurality of magneticelements, further comprising associating transition data with respectivemagnetic elements in accordance with the order in which the magneticelements transition in response to the rotating field.
 15. A methodaccording to claim 1, comprising determining the coercivity, the localmagnetic field bias resolved in the direction of each magnetic elementand the orientation of each magnetic element relative to a knowninterrogation field reference frame.
 16. A method according to claim 1,further comprising determining the amplitude response of each magneticelement to the applied magnetic field.
 17. A method of distinguishingbetween a plurality of magnetic elements, comprising the steps of:applying a scanning magnetic field to the magnetic elements; determiningthe direction of each of the magnetic elements; for each of the magneticelements, determining the components of the field in the direction ofthe magnetic element at which the magnetic element switchesmagnetisation states; and from said components, determining, for each ofthe magnetic elements, respective characteristics of the magneticelement.
 18. A method according to claim 17, comprising determiningfirst and second switching components as the components when the rate ofchange of the field along the direction of the magnetic element ispositive and negative respectively.
 19. A method according to claim 17wherein the respective characteristics comprise the coercivities of themagnetic elements.
 20. A method according to claim 17, comprisingstoring data by reference to the respective characteristics of themagnetic elements.
 21. A method according to claim 20, wherein data isstorable by reference to any one or more of orientation of the magneticelements, coercivity, bias field along the magnetic element andamplitude response.
 22. A method according to claim 20, wherein data isstorable by reference to parameters relating to any one or more of rateof change of applied field, perpendicular field, response time,characteristic response shape and the statistical distribution of theparameters.
 23. A method of determining, for a magnetic element, any oneor more of a plurality of characteristics comprising the coercivity ofthe magnetic element, the local magnetic field bias resolved in thedirection of the magnetic element and the orientation of the magneticelement, comprising: applying a varying magnetic field to the magneticelement; determining the direction of the magnetic element; determiningthe components of the field in the direction of the magnetic element atwhich the magnetic element switches magnetisation states; and from saidcomponents, determining the one or more characteristics of the magneticelement.
 24. A computer program, which when run on a computer, isconfigured to carry out a method of reading a magnetic tag having atleast one magnetic element comprising: interrogating the tag with ascanning magnetic field; determining transition data associated withchanges in the magnetisation state of the at least one magnetic element;associating the transition data with one or more respective elements;and for each element, determining the element direction whichcorresponds to the transition data for that element.
 25. A magnetic tagreader for reading a magnetic tag having at least one magnetic element,comprising: means for interrogating the tag with a scanning magneticfield; means for determining transition data associated with changes inthe magnetisation state of the at least one magnetic element; means forassociating the transition data with one or more respective magneticelements; and means for determining, for each magnetic element, themagnetic element direction which corresponds to the transition data forthat magnetic element.
 26. A tag reader according to claim 25, whereinthe scanning field comprises a rotating magnetic field.
 27. A tag readeraccording to claim 25, further comprising means for selecting themagnetic element direction which minimises the scatter of transitionpoint field vectors resolved along the direction of the magneticelement.
 28. A tag reader according to claim 25, wherein the transitiondata includes data defining first and second switching fields at which amagnetic element undergoes first and second transitions.